Pligg is an open source social publishing application. Initially released in 2005, Pligg enables website visitors to submit and vote on news articles, similar to Reddit and Digg. Pligg provides social publishing software that encourages visitors to register on your website so that they can submit content and connect with other users. Pligg creates websites where stories are created and voted on by members, not website editors. Use Pligg content management system to start your own social publishing community in minutes.

Features:

• Groups: Allow users to share articles with other members who share a common interest in a topic.
• Private Messaging: A social network site isn't very social without the ability to communicate with each other.
• Advanced Comment System: Not only can you comment on articles, but you can rate comments posted by other users.
• User Profiles: Each user that signs up on a Pligg site gets their own profile page.
• Module System: Add additional functionality to your Pligg site without having to alter the Pligg code.